1. Field of the Invention
The present invention relates to an engine for a motorcycle wherein a crank shaft and a counter shaft parallel to the crank shaft are supported rotatably by a crank case. The crank case including an upper case and a lower case joined to each other, and a main shaft having an axis parallel to the crank shaft and the counter shaft is supported rotatably by first and second support walls, the first and second walls being provided in the lower case spaced along the axis of the main shaft.
2. Description of Background Art
For example in Japanese Patent Publication No. Hei 2-43886, there already is known an engine for a motorcycle wherein a crank shaft and a counter shaft parallel to the crank shaft are supported rotatably between an upper case and a lower case which are joined together so as to constitute a crank case. A main shaft is supported rotatably by the lower case at a position corresponding to between the crank shaft and the counter shaft in order to shorten the center distance between the crank shaft and the counter shaft.
In the above structure wherein the main shaft is supported rotatably in the lower case of the crank case at a portion of the lower case other than the surface joined with the upper case, it is difficult to form a bearing by a pair of half portions. Therefore, according to the structure disclosed in Japanese Patent Publication No. Hei 2-43886, a through hole larger in diameter than a main gear of a maximum diameter out of plural main gears, which are provided on the main shaft so as to constitute a part of gear trains of plural shift ranges, is formed in one of a pair of support walls which one wall is located on the side where the main shaft is inserted at the time of assembly. A bearing is interposed between the main shaft and a bearing holder which is fitted and fixed into the through hole.
In the above structure wherein the through hole for fitting and fixing therein the bearing holder is formed in a support wall, it is necessary for the through hole to have a relatively large diameter. Therefore, a limit is encountered in shortening the distance between the main shaft and a shaft other than the main shaft. In addition, auxiliary devices which require bearings, such as a shift drum and an oil pump, are disposed in the lower case, so that it is difficult to shorten the center distance between the crank shaft and the main shaft. Therefore, in the engine disclosed in Japanese Patent Publication No. Hei 2-43886, power from the crank shaft is transmitted to the main shaft side through a chain. In the structure using a bearing holder, the number of parts used increases and the main shaft assembly becomes troublesome.